A question for the scope experts: Are there sacrifices made in scope performance, when choosing a higher magnification ratio? Does a 5-25x make compromises in any performance aspect; like eye relief, exit pupil (or eye box, if those are interchangeable), and light transmission; when compared to a 3-9x, or is everything kept in check by the typically overall-larger size of the optic? If I am missing any obvious factors, let me know; I am not an optics expert, and am trying to learn.

I’m not a scope optics expert, but a couple of thoughts.

The most obvious issue with a first focal plane scope with a large magnification range such as even 5:1 (e.g., 5-25×) is reticle design. Because the size of the reticle changes with the magnification setting, a balance must be struck between it’s being too small (thin, actually) at low magnifications and too coarse at the upper end. That’s very noticeable even in the five X range scopes I have (2-10, 3-15, 5-25, etc.), but of course if it’s 6:1, 8:1, or, mirabile dictu, 10:1, then it becomes even worse.

How important that variation in reticle size varies with the intended purpose of the sight. For scopes usually used for long range precision shooting, I’ve found that the reticle design is often optimized for the upper magnifications. It’s evidently considered less likely that such scopes will be used for aiming when set at lower powers, but rather just for increasing the field of view (assuming the dreaded “tunneling”* isn’t a feature of the sight). Therefore a thin, hard-to-see reticle at low power doesn’t matter. But that optimization isn’t always what’s followed. At full magnification, the reticle of a Leupold 5HD 3.6-18×44 is significantly coarser than that of a different brand’s 3-15×50 scope.

* As we reduce the magnification power of a variable scope, the field of view normally becomes larger, and so we have a wider view of what we’re looking at. Some scopes, however, exhibit tunneling which means that progresses only to a certain point, and then stops. For example, we have a 5-25× scope. As we decrease the magnification from 25 to 8×, the field of view widens. From 8× to 5×, however, the field of view doesn’t change; objects appear smaller and the reticle becomes finer, but the field of view remains the same. That means that that scope gives us no advantages over one whose magnification range was 8-25×, or about 3:1 rather than the 5:1 we’re paying for. An excellent video on the subject:
https://www.youtube.com/watch?v=AgTtoecSRRc

Scope manufacturers seem to have different ideas about how to deal with the reticle size problem. Most commonly it’s to have a “stepped” reticle that increases in thickness farther from the center. That permits the reticle to be seen at low powers and allows rough aiming at close distances with larger targets even if the center is very faint. Daylight bright illumination can also help.
As has been discussed here, March Optics has developed a solution that (as I understand it) essentially relies on two superimposed reticles. Because it’s been a while since I tried to understand it, though, I’ll leave it at that.

Regarding other issues, I can’t speak about current technology, but at one time the problem with “zoom” (variable focal length) photography lenses was that their optical quality was generally considered to be lower than fixed focal lenses, and the wider the zoom range, the worse the problem. With today’s high end scopes that doesn’t seem to be an issue, but even then some precision benchrest shooters prefer fixed power scopes. And if the upper magnification limit is high, there is often a desire for larger objective lenses, and the larger the lens, (traditionally) the more difficult and expensive it is to manufacture to get the same optical quality.

Eye box is another issue that I’ve only comparatively recently become aware of. A tight eye box, or “vision cone” as is more descriptive, can make a scope harder to “get behind” when shooting from improvised positions or if the shooter is old and less flexible. Unfortunately it’s not something that’s mentioned in manufacturers’ specs, and not in very many independent reviews, but it varies a lot. I haven’t experimented extensively with different magnifications, but I believe the eye box improves at lower magnifications with my scopes. It does, however, vary a lot with different brands. And BTW, eye box is not the same as exit pupil size.

Exit pupil size is usually calculated as a mathematical relationship between magnification and objective lens diameter. For example, a 7×49mm binocular has an exit pupil of 7mm, which is usually cited as the largest the average person’s pupil dilates under low light conditions. A large exit pupil (usually) permits more light to reach the eye, but if it’s larger than one’s eye pupil can dilate, then the extra may be considered a waste.* (See added comment at the bottom.) And maximum pupil size varies with individuals, and is affected by different things, including, it’s said, age.
The formula: objective lens diameter ÷ optical magnification = exit pupil size.

All that was what I learned long ago, but in recent times I’ve read stuff that complicates the issue somewhat. Independent research may be best.

And general optical quality is another huge subject. Some of that’s been discussed here by various precision shooters and others, and is worth searching for. At the same time, though, the importance of high quality glass and other features can, I believe, be overemphasized. As one reviewer pointed out, what matters is being able to see the target. Does something like color fringing matter? Maybe, maybe not. On the other hand, higher quality glass certainly helps under low light conditions. That’s why I bought a very expensive set of Zeiss binoculars when stationed in Germany long ago; what can be seen with them at night can be eye-opening (NPI ).

^ A little of what you’re asking about.

Added about first and second focal plane reticles:

As mentioned and as you may know, the reticle of a first focal plane scope changes size with the magnification setting. Its advantage is that even as it changes size, the milliradian or minute of angle calibration markings remain accurate throughout the range: a 0.1 ₥ interval of the reticle covers 0.1 milliradian on the target at 5× as well at 25×. A second focal plane reticle, however, does not change size with the magnification variation and therefore any reticle calibrations are accurate at only one magnification setting. At 20× 0.1 ₥ may cover 0.1 milliradian of the target, but at 10× it will cover 0.2 ₥ (if the magnification settings are correct).

How important is that? Although reticle calibrations can be used for range estimation, that’s not too common with the availability of laser rangefinders. More commonly calibrated reticles are used for point of aim adjustments rather than dialing them on the scope turrets. Need to hold up 2 mils? Use the 2 mark below the center of the crosshairs. Same for wind holds: wind is blowing your bullet 0.5 ₥ to the right, move the center 0.5 ₥ to the left on the target. All that’s only true, though, if the calibration markings are correct. If you have a second focal plane 20× scope set for 15× when shooting, the calibration markings won’t be accurate.

But if all that isn’t necessary for whatever reasons, especially because we’re shooting at shorter ranges on larger targets, then it does not matter. For that reason, at least one manufacturer expanded its line of mid-magnification scopes to use second focal plane reticles. Then the problems of having the reticle size become too fine or too coarse disappear. And some years ago one law enforcement sniping authority recommended using second focal plane scopes for the same reason.

* The Dark Lord of Optics has a video in which he points out why we may benefit from having an optical exit pupil that is larger than the maximum size our own pupil dilates:
https://www.youtube.com/watch?v=LiLZ_BEq7_I

This message has been edited. Last edited by: sigfreund,

That is an interesting question. Now, I am not a scope expert by any stretch of the imagination, but I've learned a few things and I thought I would share.

Let me say that sigfreund did a good job discussing the issues with reticle designs for large zoom ratios. He did mention the new design concept from March, but said he wasn't comfortable discussing it in depth. The solution that he mentions his simply a dual reticle design. There is a reticle at the FFP, which consists of a Christmas tree design and is meant to be enlarged or shrunk with the zoom. It always stays the same relative to the target in the scope. There is also another reticle, this time at the SFP. This reticle is very simple, a crosshair with a central illuminated dot. As you zoom up and down, this crosshair never changes its size, neither does the central dot. When you zoom in, the crosshair stays the same, but the tree in the FFP starts to grow and take shape on the main crosshair. It's an amazing design and it is a great solution for this issue. However, only the most advanced optical factories can produce this type of dual reticle; it has to be meticulously assembled lest you have problems with the superposition of the images. Very few companies can do this.

In my experience, the issue that is more apparent with a high zoom ratio, like 1:8 or 1:10 is something called "depth of field." The folks who see it but are not knowledgeable about optics will call touchy or shallow parallax. It's not the parallax, it's the DOF (depth of field.).

I'm going to get a little technical here, but I hope I will get the point across. As a photographer, you are aware of the DOF of your pictures and you try to work with it to produce the type of pictures that you desire. In order to use it that way, you need to understand it. When you focus a lens on a subject (or target), there is an area in front of the subject that is still in focus. There is also an area behind the subject that is also in focus. You will notice that the area behind the subject that is in focus is much larger or deeper than the area in front of subject still in focus. The distance from the nearest area that is in focus to the furthest area that is in focus is what DOF refers to.

Let's say your subject is 100 yards away. When your optics is properly focused on the subject, you will notice that starting about 40 yards down range the focus is excellent and stays that way to about 250 yards, and you are focused at 100 yards. Your DOF is 40 to 250 yards.

The DOF is a function of several components. Distance to subject, focal length, aperture (f-stop), and here comes a new concept, size of CoC (Circle of Confusion). There are many DOF calculators on-line but they are a little esoteric to use if you do not understand the various concepts.

Let's relate this back to our high zoom riflescopes. In a riflescope, the objective lens assembly will create an image at first focal plane. In a 30mm or 34mm tube size, the image at the FFP will be the size of the inner diameter of the main tube, something around 26mm. You can take a guess at the focal length of the objective lens assembly, but these figures are closely help proprietary secrets of the manufacturer. You can guess by measuring from about an inch from the front of the scope (where the objective lenses are located) to just in front of the adjustment knobs. You can calculate the aperture using the known objective lens diameter and the focal length. You know the distance to the subject. The only thing that remains is the size of the CoC, and this is where the high zoom ratios make a difference.

The CoC is essentially what defines the focus of an image. I would invite you to read about it, but essentially the smaller the CoC, the more in focus the image is. The image at the FFP is formed with the DOF included. The image will not get better from here on out.

Now the erector tube inside the main tube is the one that has the zoom lenses. At 1X zoom, the size of the image sampled by the erector tube will be the size of the inner diameter of the erector tube, Something close to 20 or 22mm. (I'm just guessing here, but I think these are good guesses.) So the image that is formed at the FFP will be sampled and not really enlarged much by the time it gets to the eyepiece; yhe eyepiece will present an image with a DOF pretty much at as it is at the FFP.

As we increase the zoom factor, the size of the image being sampled at the FFP shrinks and the resolution of the image comes more and more into play. The CoC of the image that gave us a nice big DOF at 1X zoom, is now being increased so the DOF starts to shrink. A 1X the onset of focus is 40 yards for a subject at 100 yards. At 3-4X, the onset of focus will be closer to 60 yards, and instead of 250yards being the end of focus behind the subject, that is now 200 yards. This is because the CoC is changing as we magnify the image at the FFP. At 6-7X of zoom, the DOF might now be from 75 yards to 175 yards. And at 10X zoom, the DOF might run from 80 yards to 150yards.

This is all because the magnify a progressively smaller size of the image at the FFP. I like to say that for a 1:10 zoom, the CoC needs to be .030 for 1:3X, and .020 for 4-7X and finally, .010 for 8-10X.

So you can go to your DOF calculator and uses these CoC values to determine what the DOF will be with these three values for CoC: .030, .020, and .010.

Here's a neat DOF calculator that I use:

https://dofmaster.com/dofjs.html

Thanks, NikonUser; I was hoping for your input, and now especially as you brought up a point about depth of field that I have never considered in relation to a riflescope.

It’s obvious now that you mention it because of my long experience with photography, but also because target images go in and out of focus faster at the higher powers as I adjust the parallax settings. I have one scope with a maximum of 35 power magnification, and I often use that setting to adjust the focus even if I’m going to shoot at a lower power. I did not, however, think of that factor in considering the advantages/disadvantages of different magnification ranges.

quote:

Originally posted by sigfreund:
35 power magnification

Good for keep track of your heart rate.



I swapped one rifle from a 24x to a 30x this spring and that extra 6x made my heart beat extremely more noticeable.

quote:

Originally posted by cas:
Good for keep track of your heart rate.

Yeah.

My take-aways from both of your excellent responses are:
-A higher magnification ratio makes reticles that scale with the zoom potentially not functionally ideal at the highest zoom, and almost certainly not functional at all on the lowest.
-A higher zoom ratio has a shallower depth of field range, both fore and aft of the target.
-The eye box, or vision cone, forgiving or not, is not a direct byproduct of zoom ratio.
-Nor is eye relief.

More questions:
-What does affect vision cone and eye relief, in scope design?
-What is the best way to effectively use my parallax adjustment? I understand the fundamentals of parallax, but I do not know how to best use my scope's parallax adjustment. Parallax is important to me, as my use case has me in less-than-ideal positions quite often. Does a more forgiving eye box (vision cone) mitigate the parallax effect? I found this article, which is one of the more detailed write-ups I've seen, but I don't know if it's correct; I'd appreciate your feedback.

https://www.snipercountry.com/scope-parallax/

In regards to your answers regarding the zoom ratio:
-The reticle scaling is not relevant in my use-case, as my scopes don't have that functionality; they are "SFP".
-The DoF is relevant, and seems to work in my favor, as both of my scopes are a ratio of 1:3 (or is it 3:1?).
-I figured the vision cone and eye relief would be associated with the ratio in some way; I am surprised they're not.

The reticle of a first focal plane scope may be its best size at the maximum magnification if that’s how it’s designed. I have three scopes whose reticles fit that description; in fact, even at the highest magnifications the reticle lines are pretty fine which helps with precise aiming. As I say, I suspect the scope designers assumed that they would be used at their upper magnification ranges for most applications.

But as you recognize, all that refers only to first focal plane reticles that change size with the magnification setting. For second focal plane scopes, the designers decide what’s the best size and that’s what the user gets.

I haven’t been able to find anything that explains why one scope may have a more generous vision cone (“eye box”) than another. Because it varies among different scopes, there must be some reason, but I haven’t seen any discussion of what it may be. In fact, eye box size is something that most reviewers don’t even mention. I suspect that may be because most of them either don’t have any problem getting into position properly and therefore don’t notice it. Or perhaps they haven’t ever given any thought to why one scope is easier to use than another, or maybe they always have plenty of time to get set up in position before shooting and therefore they don’t care if it’s tight.

Eye box is, however, important to some of us. I made a comment to that effect about one YouTube video and other people agreed with me for the same reasons. Some reviews will mention it. The “Dark Lord of Optics” said that the SAI Optics 6 was “easy to get behind,” and that was one of my reasons for choosing one (and later a second one—and because I really like the reticle). As I recall when I was researching Nightforce scopes there was occasional mention that they (some?) had generous eye boxes.

Eye relief is a different matter. It’s sometimes mentioned in specifications (unlike eye box), and it’s better understood in general. Years ago Leupold ran ads discussing the “optical triangle” that refers to three things in a telescope that are interrelated: magnification, field of view, and eye relief. A simple illustration:





The optical triangle isn’t something I’ve seen discussed much in recent times, but I did find this that included this quotation: Alter any one of the three sides of the Optical Triangle, and it will always affect the other two.
That means changing the magnification setting might affect eye relief, and that’s evidently why Leupold recommends mounting a scope with the magnification at maximum and with the scope positioned as far from one’s eye as possible to get a proper view.
https://mdttac.com/blog/optics...m&utm_medium=organic

Parallax adjustment. I like this guy’s videos, and here’s one:
https://www.youtube.com/watch?v=NRfWpuFTeqI

In short, though, if the parallax isn’t set properly for the target distance, if we’re not directly in proper line with the scope, the crosshairs may not be aligned where they appear to be and that can cause misses. The closer the target, the more important proper parallax adjustment is, but at the same time it’s also usually easier to detect if it’s not set properly.

The start of setting the parallax properly is to adjust the knob until the target is in sharpest focus, but keep in mind that the greater the depth of field, the harder it will be to see when the focus is sharpest, because it may be sharp for a good range of distances. The actual test for properly set parallax is to move one’s eye behind the scope without moving the scope. If the reticle seems to move with respect to the target, then the parallax isn’t set properly; adjust the knob until moving one’s eye doesn’t change the apparent position of the crosshairs.

Because the moving one’s eye test is a little fiddly, I usually adjust the focus/parallax at the scope’s highest magnification setting. Another thing to remember as a practical matter, improper parallax adjustment doesn’t matter if we’re looking straight through the scope and haven’t drifted off from the center. Parallax is also another of those things that no one really gave much thought to in the distant past. Most scopes didn’t have adjustable parallax settings and were fixed at the factory. It was common for most to be set at 150 yards, but as I recall, Leupold’s scopes intended for rimfire use were set at 75 yards, and at one time their custom shop would set the parallax at whatever distance the owner wanted. With the advent of higher magnification ranges and the desire for greater precision by shooters, manufacturers started making it possible to adjust the parallax settings ourselves.

But as a note, a more generous eye box makes it easier to not notice that we’re looking through the scope off center: Always something.

And also because I usually want to set the parallax to the proper setting without a lot of effort, some of my scopes have this aid on the adjustment dial. Although they’re not numbered, 100 yards is the red line and I know what the other lines mean. I prepared those aids after checking the parallax settings for different measured distances with the individual scopes.





Some parallax adjustment knobs have distances marked, but it’s generally recognized that they may not be very accurate. The scope that tape is on doesn’t list any distances at all.

The article you linked seemed to have good information.

This message has been edited. Last edited by: sigfreund,

quote:

Originally posted by KSGM:
My take-aways from both of your excellent responses are:
-A higher magnification ratio makes reticles that scale with the zoom potentially not functionally ideal at the highest zoom, and almost certainly not functional at all on the lowest.
-A higher zoom ratio has a shallower depth of field range, both fore and aft of the target.
-The eye box, or vision cone, forgiving or not, is not a direct byproduct of zoom ratio.
-Nor is eye relief.

More questions:
-What does affect vision cone and eye relief, in scope design?
-What is the best way to effectively use my parallax adjustment? I understand the fundamentals of parallax, but I do not know how to best use my scope's parallax adjustment. Parallax is important to me, as my use case has me in less-than-ideal positions quite often. Does a more forgiving eye box (vision cone) mitigate the parallax effect? I found this article, which is one of the more detailed write-ups I've seen, but I don't know if it's correct; I'd appreciate your feedback.

https://www.snipercountry.com/scope-parallax/

In regards to your answers regarding the zoom ratio:
-The reticle scaling is not relevant in my use-case, as my scopes don't have that functionality; they are "SFP".
-The DoF is relevant, and seems to work in my favor, as both of my scopes are a ratio of 1:3 (or is it 3:1?).
-I figured the vision cone and eye relief would be associated with the ratio in some way; I am surprised they're not.

I want to make a slight adjustment to the point about the DOF. A high zoom ratio will have a shallower DOF at the high end of the range, compared to a riflescope with a smaller zoom ratio but with a similar high magnification.

For example, a 2.5-25X scope will have a shallower DOF at 25X compared to a 5-25X scope or a 6-24X scope.

Remember that a riflescope's magnification is determined by dividing the focal length of the objective lens group by the focal length of the eyepiece group. Then you multiply by the zoom factor and that's when the CoC comes into play with the DOF at the high end.

That parallax video was very informative; about all you need to know, presented in a way that's easy to understand. Thank you for that.

Where the optical triangle is concerned: is the magnification factor referring to the ratio, in a way, as opposed to overall max magnification? It was already mentioned that, when in pursuit of high magnification, a fixed power scope perhaps allows the user to maintain some desirable aspects at high power, that a variable power scope wouldn't have, when used at high power. That, representing the extreme 1:1 ratio, seems to imply that a lower ratio may also retain more advantages from the other two sides of the triangle. Is there a graduation of sacrifices from the eye relief and FOV sides of the triangle, as we increase our ratio, or as we increase the overall maximum magnification in a variable power scope?

Also, on the subject of parallax adjustment mechanics: how consequential is the "slop" or "backlash" in the parallax adjustment, as mentioned in that article I previously linked?

quote:

Originally posted by KSGM:
Also, on the subject of parallax adjustment mechanics: how consequential is the "slop" or "backlash" in the parallax adjustment ...?

I believe that probably depends on the scope, i.e., its quality in that regard. One discussion I read a number of years ago recommended that when adjusting the parallax knob, we dial to maximum distance then back down to where we want (or think we should) be. It further stated that if we went past the desired point, we should start over from max distance. I admit, though, that I don’t do that with my scopes. I assume that because of their quality that shouldn’t be necessary, but I should probably check some time. And that’s probably the key to answering the question about any scope.

I’m not sure I understand all your questions about the optical triangle, and probably couldn’t answer them anyway because I doubt I know some of what you’re getting at.

As a general comment, though, the above statement about how changes in one affect the other two is the key point. For example, the article I linked mentioned that something like a spotting scope can maximize magnification and field of view because eye relief can be minimized.

If I were comparing two scopes’ identical magnification settings and I wanted max eye relief, I’d pick the one with the smaller field of view. If, however, I wasn’t too concerned about eye relief because I would be using it on a rifle without much recoil, then choosing a larger field of view might be better.

But to return to the original claim about the optical triangle, I believe that it deals with absolutes, not ratios within a particular instrument.

If we have any two scopes with the same field of view, for example, and one has higher magnification than the other, then it (the higher) will necessarily have shorter eye relief. I once owned an astronomical telescope that allowed me to resolve the legs of people hiking on a mountain top some 10 miles away: its magnification was very high (and its optics were very good). On the other hand, it was easy to get oil on the eyepiece lens from one’s eyelashes: extremely short eye relief. And the field of view? Relatively large for a stargazing telescope because it wasn’t that powerful as compared with some, but it was far more restricted than my 20× spotting scope’s.

In other words, I don’t believe that the ratios of a variable optic have anything to do with the question. But perhaps someone who is more knowledgeable can give a more definitive answer.

The wider the magnification range, the more optical tradeoffs there are.

A fixed 25x scope will have better performance at 25x than a 15-25x which will have better performance than a 5-15x.

Dark Lord of optics on youtube has some really good videos about optical systems in rifle scopes.

It takes more lenses for optical correction the wider the magnification range. So a scope with a wider magnification will necessarily get heavier and longer as you add more glass.

That's why the Vortex Razor has both excellent optical properties from 1x (virtually a red dot with no edge distortion), to 6x; and it is a heavy bitch because it takes a lot of lenses to work well with a range from 1-6.

Basically you look at the multiplier it takes to get from the low end to the high end.

3-9x has a multiplier of 3. It's not hard to correct with a multiplier of 3.

A 3-18 will take a lot of engineering and glass to accomplish a good multiplier of 6

It's also why LPVOs all have a 24mm objective lens. It takes bigger internal lenses to correct the light coming in from a bigger objective lens. A LPVO with a 1-6 magnification range and a 40 mm objective lens would necessitate larger internal lenses which would make it even heavier.

Basically, there is no free lunch.

Which is why sub $200 4-24 scopes like those from BSA can look halfway decent at 8x but if you crank it up to 24x the image quality goes to shit pretty quickly.

Good video on the topic:

https://www.youtube.com/watch?v=9E7W3NbhvIU

Another good video, more dealing with magnification than objective lens size.

https://www.youtube.com/watch?v=mvWklW69TBE

Thanks for all those video recommendations.
I was familiar with DLO's reviews, but not his explanations of optical systems.

Added: And another one discussing field of view.
https://www.youtube.com/watch?v=8kY-3hKZnGE

With my destroyed hearing and his delivery and, to a degree, his accent, I must listen carefully to follow everything, but it is definitely worth it.

Don’t you just love when sigfreund and NikonUser answer a question about optics?
They both start off saying “I’m no expert but…”, and then I have to spend the rest of the night studying something that amounts to a dissertation. Maybe you aren’t experts but I always learn something. Thanks guys!

quote:

Originally posted by 400m:
Don’t you just love when sigfreund and NikonUser answer a question about optics?
They both start off saying “I’m no expert but…”, and then I have to spend the rest of the night studying something that amounts to a dissertation. Maybe you aren’t experts but I always learn something. Thanks guys!

I understand and appreciate what you are saying. But I can assure you I'm in no way an expert in optics. DEON has released and started shippping their latest product, the March-X 8-80X56 HM WA Majesta riflescope. Yep, another one with a 1:10 zoom ratio.

I have been somewhat closely associated with that product in terms of specs and testing and feedback for the last 3 years. I learned a great deal about the optical design of the scope, but the most important thing that I learned is that I know virtually nothing about optical design. The optical designer has been working on this specific design for over 3 years and the calculations and various intricate parts are mind boggling.

People have started receiving their long-awaited Majestas and have been reporting that it is almost magical. They are shooting at the 1000 yards target at 80X for the entire match and they are astounded at the clarity and resolution. The FOV of the scope is the widest in the industry for an SFP design and the second widest for all designs (SFP and FFP). I used mine a couple weeks ago for my first match at 1000 yard using the specifically designed F-Class reticle and it was awesome.

I can see it, I think I understand a few things inside of it, but I still can't believe it.

Another recent video about parallax adjustments that includes a bit of discussion about the practice of going past the proper setting and then dialing back to avoid things shifting under recoil.

https://www.youtube.com/watch?v=-t6sdMTc780

I do have some question, though, about his statement that improperly set parallax is a major cause of poor precision. I would think that if it were such a contributor to problems with obtaining good groups, it’s something other top tier shooters would mention, and I do not recall anyone else ever making the same claim.

And thanks for the thanks about my contributions. I do actually know that I go on and on, but I believe that if I'm going to respond to a question it should be comprehensive within the limits of my knowledge.

The parallax is issue is one that gets overblown a lot and people use that as a crutch in lieu of blaming their marksmanship.

The knob usually on the left of the riflescope is a side focus knob though many people call it a parallax adjustment. Even some optics company call it a parallax adjustment knob because they gave up trying to educate the masses. I'm not giving up.

I suspect most people understand how parallax occurs, which is because the plane of the reticle is not in the plane of the objective. The side focus knob actuates a lens group in front of the FFP but way behind the objective group. This lens is called the focusing lens and its purpose is to focus the image of the target properly, which happens to be the FFP or the front of the erector tube assembly. This focusing is exactly the same as when you focus your DSLR.

We all remember that when you focus a lens, you are dealing with the DOF of the image. At longer distances and lower magnifications, you will get a deeper DOF that can fool you into thinking you are properly focused on the target but in fact you are not, you're just happy with the focus that you have within the DOF. The target plane is not superposed on the FFP plane, but the depth of the DOF is such that you do not see that you're not a exact focus. That's when you get into parallax issues: low magnification and longer distances, but still a very nicely focused image.

The problem with that combo is that usually the exit pupil is large enough for you to be able to see the target well without being properly behind the riflescope (sloppy marksmanship) and then you blame parallax.

As magnification increases and the exit pupil decreases, you have to get behind the riflescope better lest you see nothing in the riflescope. As you get properly behind the rifle to enjoy the full image of the target at high magnification, the parallax issues disappear. On top of that, the DOF rapidly becomes shallower as the magnification increases, so you play with the side focus more to get a properly focused image, which has the effect of removing any parallax issue, that you can even see in the first place because you have to be extremely precise in your placement behind the riflescope.

If you're shooting at 500 yards with a scope set at 8X, your DOF will be huge and you can also be sloppy behind the riflescope, which you have difficulty focusing perfectly because of the huge DOF. That's when people move their head looking for parallax, to help them focus on the image of the target. This is how optical rangefinders work. This is how rangefinder cameras used to work. You focused the lens until parallax eliminated and at that point, the lens was focused properly.

Another photographer trick with a zoom lens, is to focus on the subject at the highest magnification and then zoom down to the magnification you want to use. This is something you could do with your riflescope, go to max, focus, and come back to your desired magnification. Or play optical range finder. Or best, improve your marksmanship so that parallax is never an issue, and you just get the quickest focus you can and damn the DOF.

If you ever go to an F-class competition, you will see shooters who get perfectly behind their rifles all the time, or else they don't score worth a dam. It's a self-correcting issue.